Terrestrial digital TV broadcast is gaining popularity year after year and expected to give a nationwide coverage by 2010.
Transmitters and receivers for terrestrial digital TV broadcast have an antenna for transmission/reception of radio signals at about 600 MHz to about 1 GHz to about 2 GHz (overlapping or below the mobile phone band). Radio signals emitted from the antenna are reflected by buildings and mountains. The reflection can cause a phenomenon called multipath in which signals are delayed only temporarily, which in turn causes undesirable fading. If fading occurs frequently, it distorts frequency waveform and makes the antenna prone to error.
To prevent fading, the transmitters and receivers used currently for terrestrial digital TV broadcast employ OFDM (orthogonal frequency division multiplexing) in which, for example, a 6-mbps serial signal is converted to a parallel signal. This scheme divides a single carrier to, for example, 6000 carriers each of which is modulated. Therefore, fading, if it occurs, only affects the waveform of a particular carrier; it does not affect most carriers.
However, since OFDM requires multiple carriers, the electric power peaks of these carriers, when they come in phase, produce a very high electric power peak as shown in FIG. 20. This phenomenon is called “PAPR.” Therefore, the maximum tolerance frequency is exceeded where electric power peaks are in phase like this.
Known transmitter/receiver technology of preventing the PAPR is CI-OFDM (Carrier Interferometry OFDM or simply “CI”). See non-patent documents 1, 2. CI restrains maximum peaks by shifting the phases of the electric power peaks of divided carriers. Using CI, the amplitude is reduced by a factor of ⅓ to ¼ as shown in FIG. 21. In FIG. 21, the vertical axis indicates voltage, and the horizontal axis indicates time.
Next, a specific implementation of CI-OFDM will be briefly explained. FIG. 22 shows a conventional CI transmitter. As shown in the figure, CI includes a serial-to-parallel converter section (S/P section; serial/parallel section) 150, a modulator section 151, an IFFT (Inverse Fast Fourier Transform) section 152, a guard interval input section (GI input section) 153, and an antenna 154. Data goes through these sections in this order.
The serial-to-parallel converter section 150 distributes incoming data to multiple carriers. Here, data is divided among four carriers. The modulator section 151 has a CI (CI1, CI2, CI3, CI4 . . . ) section arranged to form a 4×4 matrix and SUM sections each subsequent to a different row of CIs. Each CI is assigned a code set and modulates a carrier. Each SUM sums the carriers modulated by CI1, CI2, CI3, and CI4.
More specifically, the carrier with bit number 1 supplied from the serial-to-parallel converter section 150 is modulated by CI1 and fed to SUM where the carrier stands by. The carrier with bit number 2 supplied from the serial-to-parallel converter section 150 is modulated by CI2 and fed to SUM where the carrier stands by. Further, the carrier with bit number 3 supplied from the serial-to-parallel converter section 150 is modulated by CI3 and fed to SUM where the carrier stands by. The carrier with bit number 4 supplied from the serial-to-parallel converter section 150 is modulated by CI4 and fed to SUM where the carrier is summed with the standing-by carriers.
The sum signal is subjected to an Inverse Fast Fourier Transform by IFFT 152. The GI input section 153 feeds GI to the signal. The resultant signal is sent out from the antenna 154.
The CIk code set for the modulation of bit number k is given by Equation 1:
                              C          ⁢                                          ⁢                      1            k                          =                  {                                    ⅇ                                                j                  ⁡                                      (                                          2                      ⁢                                              π                        /                        N                                                              )                                                  ·                0                ·                k                                      ,                          ⅇ                                                j                  ⁡                                      (                                          2                      ⁢                                              π                        /                        N                                                              )                                                  ·                1                ·                k                                      ,                          ⅇ                                                j                  ⁡                                      (                                          2                      ⁢                                              π                        /                        N                                                              )                                                  ·                2                ·                k                                      ,            …            ⁢                                                  ,                          ⅇ                                                j                  ⁡                                      (                                          2                      ⁢                                              π                        /                        N                                                              )                                                  ·                                  (                                      N                    -                    1                                    )                                ·                k                                              }                                    [                  Equation          ⁢                                          ⁢          1                ]            
Each element in Equation 1 indicates a code set. When CI is a N×N matrix, its code set is given by Equation 2:
                              CI                      N            ×            N                          =                  (                                                    1                                            1                                            …                                            1                                            1                                                                    1                                                              ⅇ                                      j                    ⁢                                                                  2                        ⁢                        π                                            N                                        ⁢                                          1                      ·                      1                                                                                                  …                                                              ⅇ                                      j                    ⁢                                                                  2                        ⁢                        π                                            N                                        ⁢                                                                  (                                                  N                          -                          2                                                )                                            ·                      1                                                                                                                    ⅇ                                      j                    ⁢                                                                  2                        ⁢                        π                                            N                                        ⁢                                                                  (                                                  N                          -                          1                                                )                                            ·                      1                                                                                                                          ⋮                                            ⋮                                            ⋱                                            ⋮                                            ⋮                                                                    1                                                              ⅇ                                      j                    ⁢                                                                  2                        ⁢                        π                                            N                                        ⁢                                          1                      ·                                              (                                                  N                          -                          2                                                )                                                                                                                        …                                                              ⅇ                                      j                    ⁢                                                                  2                        ⁢                        π                                            N                                        ⁢                                                                  (                                                  N                          -                          2                                                )                                            ·                                              (                                                  N                          -                          2                                                )                                                                                                                                          ⅇ                                      j                    ⁢                                                                  2                        ⁢                        π                                            N                                        ⁢                                                                  (                                                  N                          -                          1                                                )                                            ·                                              (                                                  N                          -                          2                                                )                                                                                                                                                1                                                              ⅇ                                      j                    ⁢                                                                  2                        ⁢                        π                                            N                                        ⁢                                          1                      ·                                              (                                                  N                          -                          1                                                )                                                                                                                        …                                                              ⅇ                                      j                    ⁢                                                                  2                        ⁢                        π                                            N                                        ⁢                                                                  (                                                  N                          -                          2                                                )                                            ·                                              (                                                  N                          -                          1                                                )                                                                                                                                          ⅇ                                      j                    ⁢                                                                  2                        ⁢                        π                                            N                                        ⁢                                                                  (                                                  N                          -                          1                                                )                                            ·                                              (                                                  N                          -                          1                                                )                                                                                                                          )                                    [                  Equation          ⁢                                          ⁢          2                ]            
Another code set, PO-CI, is also known which has a greater capacity than CI. PO-CI includes the CI code set (first code set; see Equation 3) and an additional code set (second code set; see Equation 4).
                              C          ⁢                                          ⁢                      1            k                          =                  {                                    ⅇ                                                j                  ⁡                                      (                                          2                      ⁢                                              π                        /                        N                                                              )                                                  ·                0                ·                k                                      ,                          ⅇ                                                j                  ⁡                                      (                                          2                      ⁢                                              π                        /                        N                                                              )                                                  ·                1                ·                k                                      ,                          ⅇ                                                j                  ⁡                                      (                                          2                      ⁢                                              π                        /                        N                                                              )                                                  ·                2                ·                k                                      ,            …            ⁢                                                  ,                          ⅇ                                                j                  ⁡                                      (                                          2                      ⁢                                              π                        /                        N                                                              )                                                  ·                                  (                                      N                    -                    1                                    )                                ·                k                                              }                                    [                  Equation          ⁢                                          ⁢          3                ]                                          C          ⁢                                          ⁢                      2            k                          =                  {                                    ⅇ                              j                ⁢                                  {                                                                                    (                                                  2                          ⁢                                                      π                            /                            N                                                                          )                                            ·                      0                      ·                      k                                        +                                          0                      ·                      Δθ                                                        }                                                      ,                          ⅇ                              j                ⁢                                  {                                                                                    (                                                  2                          ⁢                                                      π                            /                            N                                                                          )                                            ·                      1                      ·                      k                                        +                                          1                      ·                      Δθ                                                        }                                                      ,            …            ⁢                                                  ,                          ⅇ                              j                ⁢                                  {                                                                                    (                                                  2                          ⁢                                                      π                            /                            N                                                                          )                                            ·                                              (                                                  N                          -                          1                                                )                                            ·                      k                                        +                                                                  (                                                  N                          -                          1                                                )                                            ·                      Δθ                                                        }                                                              }                                    [                  Equation          ⁢                                          ⁢          4                ]            
The second code set is adapted so that it shows peak electric powers between adjacent peak electric powers of the first code set as shown in FIG. 23.
PO-CI boasts a double throughput of CI and smaller PAPR than CI.
However, the transmitters/receivers based on CI or PO-CI technology require complex calculations and are not easy to design. Such transmitters/receivers are theoretically possible, but not feasible yet. Therefore, no commercial products have been developed based on CI or PO-CI.    Non-patent Document 1: D. A. Wiegandt and C. R. Nassar, “High-performance OFDM via carrier interferometry,” in Proc. IEEE Int. Conf. 3rd-Generation Wireless and Beyond, 3G Wireless '01, San Francisco, Calif., 2001, pp. 404-409    Non-patent Document 2: D. A. Wiegandt, C. R. Nassar, and Z. Wu, “Overcoming peak-to-average power ratio issues in OFDM via carrier interferometry codes,” in Proc. IEEE Vehicle Technology Conf., Atlantic City, N.J., 2001, pp. 660-663